1. Field of the Invention
The present invention relates to a display device of a television receiver and, more specifically, to a liquid crystal display device employing a liquid crystal panel.
2. Description of the Background Art
CRT (Cathode-Ray Tube) displays have been widely used as display devices for television receivers. However, the depth of a CRT display can not be reduced due to the structural limit, and when the screen size is enlarged, the weight of the display is increased remarkably for implosion protection. Therefore, liquid crystal display devices having various applicability have been developed to take the place of CRT displays. A liquid crystal display device employs a liquid crystal panel in a video displaying portion thereof.
Light transmittance of a liquid crystal changes dependent on the magnitude of the voltage applied thereto. Therefore, when a large number of minute transparent electrodes are arranged on a liquid crystal panel, in which a light source is arranged on one side of a liquid crystal panel and liquid crystal is sandwiched by glass plates and the like, and various voltages are applied thereto, arbitrarily images can be formed on the panel with the brightness/darkness thereof dependent on the voltages applied thereto. Accordingly, desired images can be reproduced by applying voltages corresponding to the brightness of each portion of the images to be reproduced to the electrodes at corresponding positions on the panel. The liquid crystal display device utilizes this idea.
FIG. 11A is a schematic block diagram showing one example of a structure of a conventional liquid crystal display in a television receiver. Referring to the figure, the liquid crystal display device comprises a liquid crystal panel 61 displaying received images; a segment driver 62 and a scan driver 63 for applying signal voltages to the liquid crystal panel 61 for driving the same; a polarity inverting circuit 68 for inverting polarity of a received video signal; and a timing control circuit 69 for controlling the operation timings of the segment driver 62, the scan driver 63 and of the polarity inverting circuit 68.
In the liquid crystal panel 61, liquid crystal is sandwiched by glass plates or the like, and the panel is divided into pixels by a number of minute electrodes arranged in horizontal and vertical directions to form a matrix. FIG. 11B is a partial schematic diagram showing the structure on the liquid crystal panel 61. Referring to the figure, the liquid crystal panel 61 has M data signal lines 64 positioned parallel to each other in a vertical direction, and N scan signal lines 65 arranged parallel to each other in the horizontal direction. In the figure, each of the M.times.N portions on the panel surrounded by the data signal lines 64 and the scan signal lines 65 is a pixel. A pair of pixel electrodes 67 sandwiching the panel (the rear surface of the liquid crystal 61 is omitted) are provided for each pixel. A transparent electrode is used as the pixel electrode 67. In addition, a switching element 66 is provided for each pixel on a liquid crystal panel 61, so that the liquid crystal panel 61 is a so-called active matrix LCD (Liquid Crystal Display). The switching element 66 is provided between the corresponding pixel electrode 67 and the corresponding data signal line 64 with the ON/OFF controlled by a signal applied to the corresponding scan signal line 65. More specifically, when the corresponding switching element 66 is ON, the voltage on the corresponding data signal line 64 is applied to the pixel electrode 67. Consequently, a voltage is applied to the liquid crystal sandwiched by the corresponding pixel electrode pair, and the intensity of the transmitted light of the liquid crystal at that portion changes dependent on the applied voltage.
N scan signal lines 65 from the liquid crystal panel 61 are connected to the scan driver 63. The scan driver 63 drives the signal lines 65 by the same period as the horizontal scanning period of the received video signals, in response to a driver controlling signal from the timing control circuit 69. More specifically, it successively applies in the said period to each of the N scan signal lines 65 a signal for turning the corresponding switching means 66 ON (in the following, the scan signal line to which such a voltage is applied is referred to as a selected scan signal line).
M data signal lines 64 from the liquid crystal panel 61 are connected to the segment driver 62. The segment driver 62 drives the signal lines 64 in the same period as the horizontal scanning period of the received video signals, in response to the driver controlling signal from the timing control circuit 69. More specifically, it samples the video signal inputted through the polarity inverting circuit 68 in the said period, and internally transfers and outputs the same to apply to the corresponding data signal lines 64 successively. The timing control circuit 69 receives a synchronizing signal separated from the received video signal, and applies corresponding driver controlling signal and polarity inverting timing control signal to the segment driver 62 and the scan driver 63, and to the polarity inverting circuit 68, respectively.
The polarity inverting circuit 68 inverts the polarity of the received video signal and applies the same to the segment driver 62 in a period corresponding to the polarity inverting timing controlling signal from the timing control circuit 69. This is done from the following reason. Generally, an alternating voltage must be applied for driving the liquid crystal. Therefore, by inverting the polarity of the video signal voltage in prescribed period to apply the same to the liquid crystal, the polarity of the voltage applied to the same portion of the liquid crystal in the liquid crystal panel is changed every time, whereby the voltage applied to the liquid crystal is turned into an alternating voltage.
The operation of the liquid crystal display device will be described in the following.
The received video signal including synchronizing signal is applied to the polarity inverting circuit 68, while the synchronizing signal is separated therefrom to be applied to the timing control circuit 69.
The timing control circuit 69 forms the driver controlling signal and the polarity inverting timing control signal from the inputted synchronizing signal to output these signals to prescribed functional portions mentioned above.
In the a polarity inverting circuit 68, polarity inverting process for the inputted video signal is carried out in a prescribed period in synchronization with the polarity inverting timing controlling signal. Therefore, the polarity inverting process and the polarity non-inverting process (in which inversion is not carried out) are carried out alternately in response to the polarity inverting timing controlling signal. The inverted or non-inverted video signal is applied to the segment driver 62.
The segment driver 62 and the scan driver 63 both operate in synchronization with the driver controlling signal.
The scan driver 63 operates in synchronization with the driver controlling signal. Therefore, the scan driver 63 selects the N scan signal lines 65 starting from the upper portion one line in every horizontal scanning period of the received video signal successively and repeatedly.
Meanwhile, the segment driver 62 carries out the following operation in synchronization with the driver controlling signal. Namely, in every horizontal scanning period of the received video signal, it samples M signal voltages corresponding to each of the M pixels of one row of the liquid crystal panel 61, out of the inputted video signals of one horizontal scanning period, and the sampled signal voltages are transferred and outputted to the corresponding data signal line. It goes without saying that the switching means of each of the corresponding pixels must be ON when these M signal voltages are applied to liquid crystal of each of the pixels of the liquid crystal panel 61 through the corresponding data signal line. Therefore, each of the M signal voltages is applied to each of a row of pixel electrodes provided corresponding to the scan signal line selected by the scan driver 63 at that time. Consequently, video signals of 1 horizontal scanning period out of the received video signals are reproduced by the row of pixels. Meanwhile, the selected scan signal line is successively shifted at every horizontal scanning period by the scan driver 63. The video signals inputted to the segment driver 62 are the signals provided by successive scanning of the screen in the horizontal direction in the transmitting side, which are serially continuous signals. Therefore, when the above described operation of the segment driver 62 is repeated for N times, the received video signals of 1 field provide an image display on the liquid crystal panel 61.
Generally, when television images are transmitted, interlace scanning such as shown in FIG. 12 is carried out. FIG. 12 illustrates the interlace scanning. Referring to the figure, a television screen 71 on which the images to be transmitted are displayed is scanned along 2n-1 scan lines in total represented by solid lines and dotted lines. The numbers (1 to 2n-1) allotted to the scanning lines in the figure represent the order of scanning of the scanning lines in the actual scanning. In this manner, in actual scanning, the horizontal scan lines on the television screen 71 are not scanned one by one starting from the upper portion but every other scan lines (scan lines represented by solid lines in the figure) are scanned starting from the upper portion first. Thereafter, horizontal scan lines (represented by dotted lines) between the previously scanned lines (solid lines) are scanned successively. Namely, one image plane, that is, 1 frame is scanned by two times of scanning. Consequently, in the interlace scanning, video signals of 1 field formed by the second scanning are transmitted serially following the video signals of 1 field formed by the first scanning. In order to accurately reproduce the original images from such video signals, the image reproducing process of the receiver receiving these signals should be as follows. Namely, rough images constituted by half of the scan lines of the receiving apparatus on a display screen of the receiver side must be formed based on the video signals provided by the first scanning of the transmitting side. Thereafter, images constituted by the remaining half of scan lines must be formed based on the video signal provided by the second scanning of the transmitting side. On this occasion, when the second image reproduction is carried out, scanning is done between each of the scan lines scanned during the first reproduction. Namely, the transmitted video signals of one image frame are reproduced by these two image reproduction processes. When the display device of the receiver is a CRT display, this method is employed in accordance with the NTSC (National Television System Committee) specification and the number of valid scan lines appearing on the screen is 440 to 480 in accordance with the NTSC specification.
Now, in a receiver employing the liquid crystal display device, the number of scan lines, that is, the number of pixel rows is 220 to 240 (hereinafter this number is represented as N.sub.H) at present. In addition, in the image reproduction process of such a receiver, the scan signal lines provided corresponding to said scan lines are successively selected one by one starting from the upper portion, and video signals are applied to the corresponding pixel rows. Therefore, there are the following problems when video signals of the interlace specification are received by a receiver employing a conventional liquid crystal display device.
Since the number of scan lines of the display device in the receiver side is half of the number of scan lines in the transmitting side, the positional relation between the received images (1 frame in the transmitting side) reproduced by two image reproduction processes differs from the original positional relation scanned in the transmitting side. FIG. 13 shows the image reproduction process in a television receiver employing the currently available liquid crystal display device. Description will be given in the following with reference to the figure, in which successive numbers are allotted to the scan lines starting from the upper portion of the screen.
The correspondence between the video signals provided by scanning even numbered scan lines and provided by scanning odd numbered scan lines in the transmitting side and the scan lines on the display screen on the receiving side on which the signals are reproduced is as shown in the figure. Namely, video signals provided from each of the odd numbered scan lines Od of the transmitting side successively correspond to the scan lines 82 on the display screen 81 in the first image reproduction in the receiving side. The video signals provided from the even numbered scan lines Ev of the transmitting side successively correspond to the scan lines 82 on the display screen 81 in the second image reproduction. Therefore, the odd numbered scan lines Od and the even numbered scan lines Ev of the transmitting side, which should appear alternately on the receiving apparatus, overlap with each other at the same position, as shown in the figure.
Consequently, vertical resolution of the reproduced image in the receiving side is considerably inferior to that of the images (original images) of the transmitting side. In other words, although video signals are formed with high fidelity to the original images by scanning 440 to 480 scanning lines, video signals of 1 field of the original images are reproduced by 220 to 240 scan lines in the receiving side, so that only rough images can be provided. In order to solve such a problem, the number of scan lines of the display screen (liquid crystal panel) of the liquid crystal display device, that is, the number of scan signal lines, must be increased to be approximately the same as that of the CRT display.
However, in the conventional liquid crystal technique, it was difficult to produce a liquid crystal display screen having so many scan signal lines. Further, even if such a liquid crystal display screen was produced, there were various problem in driving the same. Recent development in the liquid crystal technique seems to enable actual application of the above described liquid crystal display screen.
However, the following problem is left unsolved when the number of scan signal lines on the display screen are simply increased than that of the conventional device. When the interlace scanning, as in the CRT, is to be simply carried out on the liquid crystal display device, the following method may be used. Namely, the conventional method in which scan signal lines are successively selected one by one from the upper portion in the conventional liquid crystal display device is changed such that every other scan signal lines are scanned first starting from the upper portion, and the scan signal lines not selected in the first scanning are selected in the second time, also starting from the upper portion. Namely, the odd numbered scan signal lines are successively selected according to the order, and the even numbered scan signal lines are successively selected according to the order. However, in this method, there are always non-selected scan signal lines during scanning, and application of video signals are not carried out in the pixel electrodes corresponding to the non selected signal lines. Therefore, previously reproduced information are continuously displayed on the pixels corresponding to the non selected signal lines. When images with rapid movement are to be reproduced, accurate reproduction cannot be expected.